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Comment by phkahler | Hacker News Reader

>> The innovation rate in Fusion is way too slow...

I'm starting to think real progress on fusion is suppressed because there might be a risk lurking there.

If you want tinfoil theories, my theory is that the massive lobbying power of the fossil fuel interests delay R&D and implementation of alternative energy. And as some of the alternative energies are getting momentum, like wind&solar, they are also lobbying against potential competitors.

As far as I understand there is a nuclear waste problem, since neutrons are generated which activate the hull significantly. Neutron cannot be deflected.

But I dont believe this is the show stopper why progress is slow. To me it feels more like people know ther e is a technical insurmountable problem ahead (like how to efficiently enough transform the energy to electricity; actually really wondering how this should be scalable) and people involved do not want to reach that point.

Isn't the energy converted to electricity by heating water and generating steam then doing any number of well understood things like spinning turbines?

Getting the heat from neutrons randomly flying out of the reactor to water is a hard problem. The current thought is to cool the reactor with liquid lithium, which itself will be undergoing fission and releasing helium during the process. The liquid lithium and helium would then be pumped through water cooled heat exchanger and some sort of filtering process to remove the helium. Also, all of the plumbing and materials have to withstand radiation 100 times that found in fission reactors.

I’m not saying it is impossible, but they won’t be working on it until ITERs successor sometime in the 2050s

Is there a reason graphite won't work? It's conductive, stable and extracts thermal energy from neutrons fairly reliably in fusion reactors.

I have no clue, but would love to know.

I hesitate to speculate, but one possibility is that for large reactors you have so much heat that you need heat exchange by liquids rather than solids or gases or non-atomic particles. My understanding is that solids are bad because electrons and phonons cannot carry as much energy as atoms. Gases are bad because they don't have enough atoms. This leaves liquids as best, which is why we so often see liquid cooling in animals and machinery.

So letting the lithium flowing near the plasma?

Water is fine as a cooling medium, the reason for the lithium blanket stuff is to produce tritium for fuel.

That is one proposed path; another is direct conversion of high-energy particles to electricity. The former seems much more likely to be feasible, but the latter is potentially more efficient.

What conceivable risk could there be that supports this hypothesis?

Surely with a little imagination you can conceive of some risks that might be associated with having a small sun in your backyard.

For one, fusion reactions generate neutrons that activate all the structural material of the reactor, creating radioactive waste. The waste is shorter lived than fission waste but is still a risk.

A more obvious risk is the pure destructive power of that much energy. High pressure, 100 million degree plasma is full of risks. It's nice that fusion reactors can't "run away" like fission does, but you still have to deal with material 6x hotter than the sun escaping confinement.

My comment wasn't very focused, so I'll clarify: GP seemed to imply that there is some hidden risk about fusion power that nobody is aware of or talking about, as if such a risk is the greatest factor currently bogging down fusion research. I wanted them to expand on that thought, as it's definitely not a perspective I'm familiar with.

When operational a fusion reactor has less then a couple grams of fuel in the vacuum vessel.

Okay, let's do some back of the napkin calculation.

The specific heat of Hydrogen gas is roughly 20 J/gC at high temperatures[1]. It would be much higher for a plasma but I couldn't easily find a number.

At 100M degrees C, a single gram of hydrogen would have over 2 Gigajoules of thermal energy. 2 grams of hydrogen would have the same energy as a literal ton of TNT.

It's not untenable, but its not really something that should be brushed off as "its only a couple grams of fuel" or "it can't have a runaway reaction".

[1] https://www.engineeringtoolbox.com/hydrogen-d_976.html

Having energy does not demonstrate danger to life and limb. 2 tons of fat have more energy than 2 tons of TnT. Obviously it does not release energy in the same way.

That plasma is at less than atmospheric pressure, and is contained in a massive steel vessel. Even if it expands and melts through the vessel, the atmospheric pressure will collapse it. I do not see a plausible scenario where anyone is in jured.

Funnily enough the much bigger danger is magnet quenches. Thousands of amps boiling off a couple dozen tons of liquid whatever is going to be a hell of a danger, but it's still "violent industrial accident" and not "fireworks factory" - and definitely nothing which people think of when you say "nuclear".

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